The present invention relates to an operational transconductance amplifier (xe2x80x9cOTAxe2x80x9d) which is an amplifier having a mutual conductance which can be controlled, and a filter circuit which uses the OTA. More particularly, this invention relates to the OTA and the filter circuit formed of complementary metal-oxide semiconductor (xe2x80x9cCMOSxe2x80x9d) devices.
The mutual conductance of the OTA depends on the input offset voltage. If a circuit is formed of a plurality of OTA""s, therefore, dispersion of the circuit becomes great as a result of changes in mutual conductances of the OTS""s. In order to prevent such dispersion, it is desirable to adopt a configuration in which the mutual conductance of each OTA is least affected by the input offset voltage.
Conventionally, as an OTA formed of CMOS devices, an OTA having a configuration shown in FIG. 1 is known. As shown in FIG. 1, this OTA includes three N-channel MOSFET""s (xe2x80x9cNMOS field-effective transistorsxe2x80x9d) 11, 12 and 13, four current sources 14, 15, 16 and 17, two input terminals 18 and 19, two output terminals 20 and 21, and a control voltage input terminal 22.
A first NMOS field-effective transistor 11 is connected at its gate to a first input terminal 18, to which an input voltage Vin is applied. The first NMOS field-effective transistor 11 is connected at its drain to a second output terminal 20, which outputs an output current IoutX. In addition, the first NMOS field-effective transistor 11 is connected at its drain to a power source terminal as well via a first current source 14. The first NMOS field-effective transistor 11 is connected at its source to a third NMOS field-effective transistor 13 at its source, and to ground via a third current source 16.
A second NMOS field-effective transistor 12 is connected at its gate to a second input terminal 19, to which an input voltage VinX is applied. The second NMOS field-effective transistor 12 is connected at its drain to a first output terminal 21, which outputs an output current Iout. In addition, the second NMOS field-effective transistor 12 is connected at its drain to a power source terminal as well via a second current source 15. The second NMOS field-effective transistor 12 is connected at its source to the third NMOS field-effective transistor 13 at its drain, and to the ground via a fourth current source 17. The third NMOS field-effective transistor 13 is connected at its gate to a control voltage input terminal 22, to which a control voltage Vc is applied from the outside.
In this conventional OTA, mutual conductance is controlled by adjusting the control voltage Vc and thereby changing the resistance of the third NMOS field-effective transistor 13. Mutual conductance Gm of the OTA can be represented by using a gate-source voltage Vgs and a threshold voltage Vth of the third NMOS field-effective transistor 13 and a transconductance factor K as indicated by the following equation (1).
Gm=K(Vgsxe2x88x92Vth)xe2x80x83xe2x80x83(1)
Typically, in the OTA, an input offset voltage of approximately several tens mV exists. Therefore, the voltage of the source of the third NMOS field-effective transistor 13 rises by a voltage corresponding to the input offset voltage. Denoting the input offset voltage by Voff, the equation (1) changes to the following equation (2). In other words, the conventional OTA has a problem that the mutual conductance is shifted from its preset value by xe2x88x92KVoff when the input offset voltage is Voff.                                                         Gm              =                              K                ⁡                                  (                                      Vgs                    -                    Voff                    -                    Vth                                    )                                                                                                        =                                                K                  ⁡                                      (                                          Vgs                      -                      Vth                                        )                                                  -                Voff                                                                        (        2        )            
It is an object of the present invention to provide an OTA in which the deviation of the mutual conductance caused by the input offset voltage is nearly zero, and provide a filter circuit that uses such OTA""s.
The operational transconductance amplifier according to one aspect of the present invention comprises a MOS field-effective transistor having a gate, wherein a resistance of the MOS field-effective transistor changes according to a voltage applied to the gate and transconductance is changed according to the changed resistance, a center voltage measurement circuit which measures a center voltage of two input voltages and outputs one of a voltage and a current based on the measured center voltage, and a voltage addition circuit which supplies the gate of the MOS field-effective transistor with a voltage obtained by adding the voltage or current output from the center voltage measurement circuit to a control voltage or a control current.
The operational transconductance amplifier according to another aspect of the present invention comprises a MOS field-effective transistor having a gate, wherein a resistance of the MOS field-effective transistor changes according to a voltage applied to the gate and transconductance is changed according to the changed resistance, a first voltage addition circuit which outputs one of a voltage and a current obtained by adding a voltage or current based on a first input voltage to a control voltage or a control current, a second voltage addition circuit which outputs one of a voltage and a current obtained by adding a voltage or current based on a second input voltage to the control voltage or the control current, and a center voltage measurement circuit which measures a center voltage of the voltage or the current output from the first voltage addition circuit and the voltage or the current output from the second voltage addition circuit, and which supplies a voltage based on the measured center voltage to the gate of the MOS field-effective transistor.
The operational transconductance amplifier according to still another aspect of the present invention comprises a plurality of MOS field-effective transistors each having a gate, wherein a resistance of each of the MOS field-effective transistors changes according to a voltage applied to the gate and transconductance is changed according to the changed resistance, wherein the MOS field-effective transistors being connected so that resistors having the same resistance will be connected in series according to the gate voltage, and which output one of a voltage and a current based on a center voltage of two input voltages, from a node of the resistors connected in series, and a voltage addition circuit which supplies the gates of the MOS field-effective transistors with a voltage obtained by adding the voltage or current output from the node of the MOS field-effective transistors to a control voltage or a control current.
The filter circuit according to still another aspect of the present invention comprises a plurality of operational transconductance amplifiers according to the above-mentioned aspects.
Other objects and features of this invention will become apparent from the following description with reference to the accompanying drawings.